1. Field of the Invention
This invention relates to the measurement of ocean currents and more particularly to systems of electromagnetic flow meters with improved accuracy and capability.
2. Description of the Prior Art
The problem of measuring ocean currents at various depths simultaneously in order to obtain a consistent and reliable model of water circulation, presents a challenging set of obstacles. Aside from the standard difficulties of shipboard deployment, one must deal with limited power supplies, the tendencies of materials to corrode when left for several months in contact with sea water, the physical beating which oceans apply to apparatus left therein, in electrical measurements, the substantial interferences which arise from the varied electromechanical and electrochemical activities of sea water in contact with sensors, and most importantly, the extremely weak levels of signals obtained from the relatively slow passage of sea water through a relatively low-powered magnetic field of an electromagnetic sensor.
An important problem is that the physical structure of measuring apparatus may itself deflect currents in the vicinity of measurement sensors so that subsequent data handling must include means for correcting such systematic deflections. Aside from the potential for introducing errors in the correction process, changes in flow velocities of the water may have unanticipated actions around the structure which may yield still further inaccuracies. The open cage construction of Olson's electromagnetic flowmeter, U.S. Pat. No. 3,693,440, wherein a uniform magnetic field is created over a cylinder of water by a pair of Helmholtz coils (which themselves define the cylinder) with measuring electrodes disposed within the cylinder for reading the voltage differential, illustrates one solution to the problem under discussion. Water flows freely through the cylinder substantially undistorted from the general flow of which it is a representative part, and (but for such distortion as might be introduced by the electrodes and their support apparatus) useful signals may thereby be obtained.
The electronic problems of seagoing flowmeters are also significant. Standard electrodes, for reasons inherent to the structure of various prior art flowmeters, protrude into the water stream where they not only introduce turbulence (which modifies the very flow they seek to record) but also offer plentiful opportunities for encountering aquatic life forms which may attach to the electrodes, altering their properties and even rendering them inoperative within relatively short periods.
The faces of electrodes in sea water tend to become tiny batteries as dipole molecules, attracted by unavoidable static charges, line up along their surfaces. Potentials of such batteries can be in the range of one to ten millivolts, highly variable with changes in temperature, local pressure, the chemical composition of the electrode and the sea water, and simple mechanical activity of the water against the dipole layer which lies in contact with the electrode surface. It is important that signal-processing apparatus attempting to read the electrical activity created by the passing water currents, be substantially unaffected by electroche mical potential differences which appear as a time varying DC offset. These voltages can be of similar or larger magnitudes compared with the very weak flow-generated signals which the flowmeter is to detect and report. An otherwise excellent circuit, to be discussed in greater detail below in connection with FIG. 1, which would be ideal for interpreting flowmeter data, and which is widely used for data acquisition in situations having relatively stronger input signals, has yielded disappointing results in flowmeter applications because of its substantial DC offset gain, resulting in a relatively poor signal-to-noise ratio.
To get sufficiently strong signals when using this kind of circuit, flowmeters employing it must be kept close to shore or to seaborne support apparatus so that substantial amounts of input power may be supplied. Several attempts at limiting the DC offset gain in this circuit have produced unfortunate side effects, primarily in reducing the common-mode rejection ratio and common-mode range.
These capabilities are important in an electromagnetic flow sensor not only because of DC offset problems, but also because of the relatively large electric fields and the consequent large transients developed in the system when the polarity of the magnetic field is reversed. They are also important when testing and calibrating flowmeters in tow tanks, generally located in buildings with background 60 Hz electrical activity, which, while low for most purposes, are of sufficient magnitudes to interfere with electromagnetic oceanic flow meters, considering the kinds of sensitivities they must have.
The prior are includes a widely used, very high input impedance differential amplifier utilizing a differential to single-ended converter which is schematically shown in FIG. 1. When operated at high gain, it provides a high common mode-rejection ratio (CMRR) of approximately 40+20.times.log (gain), or 120 to 140 dB for a gain of 10E4 to 10E5, together with an extremely high input impedance (10E12 ohms) and a large common-mode range (CMR) equal to only slightly less than the power supply voltage. These specifications are obtained with inexpensive low noise dual operational amplifiers. Unfortunately, this configuration exhibits very high DC offset gain which would amplify the DC offsets produced at the interface between the pickup electrodes and the sea water electrolyte, as well as offsets deriving from the amplifiers themselves.
Various means of reducing the DC offset gain in this circuit have been attempted, but they tend to result in severe degradation of the CMRR, CMR, or both. One modification of the circuit of FIG. 1 is shown in part in FIG. 2. The differential to single-ended converter of FIG. 1 is the same for all circuits shown herein, consequently, in subsequent figures it is shown with its associated components merely as a block. This modification results in loading of the electrode system by resistors 12 and 14, and capacitors 16 and 18. The 1% tolerance of resistors 12 and 14, together with the greater tolerances of capacitors 16, 18, 20, and 22 severely limit the CMRR obtainable by this circuit. Additional unbalances contributed by the unpredictable variation in electrode resistance (200-5,000 ohms) result in conjunction with the input loading effects, in a further reduction of CMRR.
This circuit eliminates the effect of electrode offset potentials, but not the effect of amplification of the input offset potentials of the operational amplifiers. This input stage must therefore operate at a total gain of only 25, which further imposes a limit on CMRR to an expected value of 68 dB, and a worst case of about 60 dB, assuming the use of 1% tolerance resistors. The combination of these limiting factors leads to an overall CMRR of less than 60 dB, which is insufficient in our experience to permit stable and reproducible measurements over the long term in the ocean.
In addition, capacitors 20 and 22 limit the bandwidth of the front end resulting in slow recovery from large transients.
FIG. 3 shows another attempt at coping with the DC offsets. An error signal developed by differential to single-ended converting amplifier 32 and amplifier 34 and an equal voltage of opposite polarity developed by amplifier 36 are applied to the negative summing junctions of input amplifiers 38 and 40 through resistors 42 and 44. This circuit substantially eliminates the effect of unbalanced electrode resistance by connecting the electrodes directly to the high impedance (10E12 ohm) inputs of amplifiers 38 and 40. This circuit requires exact matching of the resistance values of resistors 42 and 44. In a practical instrument the use of 1% resistors would result in an expected CMRR of 20.times.log (100)=40 dB, and a worst case of 34 dB. Use of the more expensive 0.1% tolerance resistors improves these values to 60 and 54 dB, which are not as good as those of the circuit of FIG. 2.